Up until now, we’ve discussed conventional oil and gas resources. These resources were the primary sources of petroleum for most of the history of the industry.
However, there has recently been a major shift towards the development of unconventional resources.
Of particular importance in the U.S. is the recent development of shale gas.
The Unconventional Petroleum System
Shale gas taps into the gas that was not expelled from the source rock at the time of generation. Although shales expel large amounts of gas during kerogen ‘cracking’ that migrate into conventional reservoir rocks, much of the gas remains trapped within the rock where it was generated. In effect, the source rock serves as its own reservoir rock. No migration pathway is needed in this unconventional petroleum system. The highly impermeable source rock serves as its own trap. Because many of these shales extend across large basins, they represent huge accumulations of gas. The trick is to get the gas to flow out of the rock – neither buoyant forces nor the influence of a water column are significant to promote flow in an unconventional petroleum system.
Shales associated with shale gas have particularly low permeabilities and do not contain enough fractures to allow the gas to move out of its source rock. Because of this, the only way to extract the trapped gas is to create an artificial network of fractures within the rock – the goal of hydraulic fracturing.
Other types of unconventional resources include tight gas and tight oil. In these resources, hydrocarbons did escape from the shale where they were generated. However, the reservoir rocks they now occupy have such low permeabilities that gas cannot be produced at a significant rate to justify the cost of drilling. Once again, hydraulic fracturing can serve to create a network of fractures through which gas can move to the wellbore more quickly.
Shale gas, tight gas, and tight oil are more common than conventional resources because they do not require the presence of a structural trap or seal. Whereas most of the conventional hydrocarbons escaped at the surface in the distant past, or has already been tapped, these unconventional resources held on to most of the oil and gas they ever produced. Because these source rocks retain large quantities of hydrocarbons and extend over large basins, the technological ability to produce these unconventional resources produced the shale gas revolution.
Petroleum geologists continue to leverage technological advances to help them find oil and gas in both conventional and unconventional reservoirs. And as the shale gas revolution has demonstrated, these advances can significantly change the global energy landscape.
Images: “Unconventional” by Jim Ladlee for Top Energy Training